Solder fill into high aspect through holes

ABSTRACT

A method for filling a through hole with solder includes mounting a substrate having a through hole formed therein on a permeable barrier layer having pores that enable gas to flow through the permeable barrier. A solder source is positioned over the through hole. Molten solder is delivered in the through hole with a positive pressure from the solder source such that gas in the through holes passes the permeable barrier while the molten solder remains in the through hole.

BACKGROUND

Technical Field

The present invention relates to integrated circuit processing, and moreparticularly to filling high aspect ratio through holes with solder.

Description of the Related Art

Injection Molded Solder (IMS) technology utilizes a local vacuumfunction under a scanning head to effectively fill molten solder into anenclosed mold space, e.g., single-end via holes. Maintaining uniform andhigh vacuum conditions is difficult due to the scanning feature of thehead. The head moves and needs to reseal to draw a vacuum over each holeto be filled. This method makes solder filling into high aspect viaholes difficult and may result in slower throughput.

SUMMARY

A method for filling a through hole with solder includes mounting asubstrate having a through hole formed therein on a permeable barrierlayer having pores that enable gas to flow through the permeablebarrier. A solder source is positioned over the through hole. Moltensolder is delivered in the through hole with a positive pressure fromthe solder source such that gas in the through holes passes thepermeable barrier while the molten solder remains in the through hole.

Another method for filling one or more through holes with solderincludes mounting a substrate having a through hole formed therein on apermeable barrier layer having pores that enable gas to flow through thepermeable barrier; pressurizing a solder reservoir; positioning ascanner head over the through hole configured to deliver molten solderfrom the solder reservoir; delivering molten solder in the through holewith a positive pressure such that gas in the through holes passes thepermeable barrier while the molten solder remains in the through hole;and repositioning the scanner head over another through hole.

A system for filling a through hole with solder includes a heatingplate, a permeable substrate mounted on the heating plate and apermeable barrier disposed on the permeable substrate. The permeablebarrier includes pores that enable gas to flow through the permeablebarrier while preventing molten solder from flowing through thepermeable barrier. The permeable barrier is configured to create aninterface with a substrate having one or more through holes formedtherein. A repositionable scanning head is configured to deliver moltensolder to the one or more through holes with a positive pressure.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a schematic diagram of a solder filing system showing across-sectional view of a substrate with through holes being filledusing a pressurized solder source in accordance with the presentprinciples;

FIG. 2 is a cross-sectional view of the substrate of FIG. 1 with throughholes filled with solidified solder in accordance with the presentprinciples; and

FIG. 3 is a block/flow diagram showing methods for filling one or morethrough holes with solder in accordance with illustrative embodiments.

DETAILED DESCRIPTION

In accordance with the present principles, systems, devices and methodsare provided for filling high aspect ratio through holes without theneed for a local vacuum. The present principles employ a pressurizedsolder flow into a through hole. The through holes receive solder on afirst end and interface with a permeable barrier on a second end. Thepermeable barrier permits gas to escape but prevents the flow of solderon a far end of the through hole. The porous or permeable barrier (e.g.,polytetrafluroethylene (PTFE)) permits permeation of air and preventspermeation of molten solder. The permeable barrier may include a thinporous PTFE film that may include sub-micron pore sizes (e.g., <1micron). The permeable barrier may be realized as a single layer ormultiple layers. In addition, the permeable barrier may be supported byadditional layers or structures.

In other embodiments, ultra-high molecular weight polyethylene (UHMWPE),high-density polyethylene (HDPE), polypropylene (PP), and polyvinylidenefluoride (PVDF) may be employed as materials for the permeable barrier.In still other embodiments, ethylene vinyl acetate (EVA),polyethersulfone (PES), polyurethane (PU) and polyethylene (PE)/PPco-polymer may also be employed.

In one embodiment, the permeable barrier is provided on a permeablesubstrate. The permeable substrate may include a ceramic material. Thepermeable substrate may include, e.g., Al₂O₃, AN, etc. The permeablesubstrate also permits air permeation and heat transfer from a heatingplate. The permeable substrate may be placed on a heated plate tomaintain solder flow temperatures at the second end to ensure a completefill throughout the through holes. The permeable barrier can eliminatesolder-contamination of the permeable substrate (ceramic plate), and thesoftness (smoothness) of the permeable barrier forms a smooth soldersurface. While a permeable substrate is preferably other textures andmaterial types may also be employed for the permeable substrate.

A molten solder filling tool in accordance with the present principlesincludes a scanning head that injects solder into the through holes. Tomake high aspect vias with solder, a substrate includes open throughholes through the material. The tool enables solder filling by employinghigher pressure than atmospheric pressure without a local vacuumfunction. Higher pressure can be applied to molten solder since there isno concern of solder leakage into the vacuum system and no fear ofsolder leakage on the far end of the through holes due to the permeablebarrier. The solder leakage into local vacuums in conventional devicescould damage the expensive vacuum system. In conventional systems, thevacuum system can only provide a maximum pressure of atmosphericpressure (1 atm) to eliminate voids in the molten solder. This is astrict limitation on the pressure differential employed in solderfilling. In accordance with the present embodiments, this pressurelimitation is not present and a much larger pressure differential may beemployed using positive pressures over atmospheric pressure.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SnPb. These compounds includedifferent proportions of the elements within the compound, e.g., SnPbincludes Sn_(x)Pb_(1−x) where x is less than or equal to 1, etc. Inaddition, other elements may be included in the compound, such as, e.g.,AuSnPb, CuSnPb, etc. and still function in accordance with the presentprinciples. The compounds with additional elements will be referred toherein as alloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a system 10 isillustratively shown provided for filling high aspect ratio throughholes without the need for a local vacuum in accordance with the presentprinciples. A solder reservoir or delivery mechanism 22 provides solderflow into a scanning head 18. The mechanism 22 may include heating formaintaining molten solder 16. The scanning head 18 may also includeheating elements or the like to maintain the flow of solder 16. Thesolder 16 may include a SnAgCu solder (SAC solder), although anysuitable solder can be employed. In one embodiment, the mechanism 22 maypressurize the solder 16 to ensure a complete fill of through holes 14on an integrated circuit device or substrate 12. It should be noted thatthe present principles are applicable to through holes with any aspectratio, but are particularly useful for high aspect ratio through holes(e.g., 1.5 or greater).

The scanning head 18 delivers solder 16 into through holes 14. Thescanning head 18 receives solder 16 through a delivery system 20. Thedelivery system 20 may include piping or other enclosed vessels tomaintain pressure. The scanning head 18 is repositionable and iscontrolled to be positioned over through holes 14 to enable filling ofthe through holes 14 with solder 16. The relative motion of the scanninghead 16 relative to the substrate 12 may be controlled by controllingmovement of the scanning head 16, by controlling the movement of thesubstrate 12 or both.

The through holes 14 are provided through the device or substrate 12.The through holes 14 include a first end in which the solder 16 isprovided for each through hole 14. The solder 16 may be pressurized todecrease fill time and to ensure a complete fill of through holes 14.The solder 16 may be pressurized e.g., using a pump or piston cylinderarrangement, pressurized gas, gravity, etc. to generate pressure withinthe molten solder 16. In one embodiment, a positive pressure 24 (asopposed to negative (vacuum) pressure) is applied that may be betweenabout 1 atm to about 5 atm. In another embodiment, the hydrostaticpressure of the solder 16 may be employed (solder column). The scanninghead 14 does not include a local vacuum although a local vacuum may beemployed in some embodiments along with other features in accordancewith the present principles to further enhance solder flow.

A permeable barrier 26 is disposed on a second end of the through holes14. The permeable barrier 26 may be pressed against the substrate 12 ormay otherwise be removably attached to the substrate 12. The permeablebarrier 26 permits gas to escape from the through holes 14 when beingfilled with solder 16. The permeable barrier 26 is in fluidcommunication with the though holes 14.

The permeable barrier 26 prevents the flow of solder 16 beyond thepermeable barrier 26. Pore size of the permeable barrier 26 is selectedso that surface tension of the solder 26 prevents the solder 26 fromentering pores of the permeable barrier 26. The porous or permeablebarrier 26 may include a polymer material, such as, e.g.,polytetrafluroethylene (PTFE) although other materials may be employed.For example, in other embodiments, UHMWPE, HDPE, PP and PVDF may beemployed as materials for the permeable barrier 26. In still otherembodiments, EVA, PES, PU and PE/PP co-polymer may be employed.

The permeable barrier 26 may include sub-micron pore sizes of, e.g., <1micron. This permits permeation of air and prevents permeation of moltensolder.

In one embodiment, the permeable barrier 26 is provided or formed on apermeable substrate 28. The permeable substrate 28 may include a ceramicmaterial, such as, e.g., Al₂O₃, AN, etc. The permeable substrate 28 alsopermits air permeation and heat transfer from a heat source 30, e.g., aheating plate. The permeable substrate 28 may be placed on the heatsource 30 or otherwise be connected to the heat source 30. The heatsource 30 provides energy to the permeable substrate 28, permeablebarrier 26 and the substrate 12 to maintain solder flow temperatures atthe second end of the substrate 12. By keeping the solder flowing, acomplete fill throughout the through holes 14 is achieved. The permeablebarrier 26 can eliminate solder-contamination of the permeable substrate28 (ceramic plate), and the softness (smoothness) of the permeablebarrier 26 forms a smooth solder surface on solidified solder throughvias. Smooth surfaces reduce whiskers and therefore reduce thepossibility of shorts, corrosion, and other ill-effects.

The scanning head 18 injects solder into the through holes 14. To makehigh aspect ratio vias with solder, scanning head 18 delivers solder 16with a higher pressure than atmospheric pressure without a local vacuumfunction. Higher pressure can be applied to molten solder 16 since thereis no concern of solder leakage into a vacuum system, and leakage of thesolder 16 can be controlled by a clamping force of the substrate 12against the permeable barrier 26.

In accordance with the present embodiments, a much larger pressure maybe employed to achieve a pressure differential of greater than betweenabout 0.1 atm and about 5 atm depending on the arrangement. This is notachievable by a conventional local vacuum system.

The scanning head 18 and the reservoir 22 maintain molten soldertemperatures, e.g., 180 to 270 degrees C. The scanning head 18 movesfrom through hole 14 to through hole 14 (e.g., in the direction of arrowA in this example) at speeds of between about 0.2 mm/sec to about 1mm/sec, preferably about 0.5 mm/sec.

Referring to FIG. 2, a device 40 is illustratively shown with highaspect ratio through holes filled with solder in accordance with thepresent principles. After the through holes (14, FIG. 1) have beenfilled with solder, the solder is solidified to form through vias 32through the substrate 12. The substrate 12 may include a semiconductorcrystal, a glass substrate, a ceramic substrate, an epoxy resin (forprinted wiring boards, etc.). The substrate 12 may include a thicknessof between about 250 microns to about 1000 microns, although otherthicknesses may be employed. The higher pressure and the heated porousbarrier (26) enable a thicker substrate 12 to be employed. In oneexample, a 500 microns substrate 12 had through vias 32 formedtherethrough with a thickness/diameter of approximately 40 microns.Other dimensions are contemplated as well.

In useful embodiments, through holes having aspect ratios of 25 orgreater can be filled with solder in accordance with the presentprinciples. The device 40 may include a plurality of differentstructures. Useful embodiments may include: high aspect ratio throughvia applications, all 3D/2.5D applications (stacked chips, interposers,etc.) (including, e.g., memory chip stacks), sensors, low powerbio-inspired devices, through silicon vias, through glass vias, throughmold vias, vias through organic substrates, etc. The present principlesare not limited to semiconductor devices and are also applicable tovarious electrical devices with electrical connections between opposingsurfaces of a substrate.

In one arrangement constructed by the present inventors, atin-silver-copper (SnAgCu, or SAC) was employed having a meltingtemperature of about 220 degrees C. The scanning scan speed employed wasabout 0.5 mm/sec with no local vacuum on the scanning head. Thepermeable barrier included a porous PTFE film which had a pore size ofabout 0.2 microns and was configured to permit air flow while blockingliquid flow (molten solder). The permeable barrier was mounted on aporous ceramic substrate (e.g., porous alumina having a porosity ofabout 60%). A 500 micron thick substrate having 40 micron diameterthrough holes were completely filled with solder. A bottom surface ofthe solder vias was uniform and included a smooth surface finish. Thesmooth surface included a finish of less than about 0.5 microns (inuseful embodiments, e.g., a surface finish of 0.05-0.20 microns RMS maybe employed).

Referring to FIG. 3, a method for filling a through hole with solder isshown in accordance with the present principles. In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

In block 102, a substrate having one or more through holes is mounted ona permeable barrier layer. The permeable barrier is porous and enablesgas to flow through the permeable barrier while blocking leakage ofmolten solder. In block 104, a solder source is positioned over athrough hole. In block 106, molten solder is delivered into the throughhole with a positive pressure from the solder source such that gas inthe through holes passes the permeable barrier while the molten solderremains in the through hole.

In one embodiment, a solder reservoir is employed to store moltensolder. The reservoir or a connection tube coupled to a scanning headmay provide pressurization of the solder. The positive pressure isgreater than 1 atmosphere.

In block 108, the permeable barrier and the substrate are heated toensure the molten solder fills the through hole. The heat ensures thatthe solder stays molten to completely fill the through hole. The heatingmay be provided by a heated plate. A permeable plate may be disposedbetween the heated plate and the permeable barrier. The permeable platefurther permits gas flow through the permeable plate.

The solder source may include a repositionable scanner head. In block110, the scanning head (solder source) may be programmed so that it ispositionable over a through hole and releases molten solder into therespective through hole. After filing the present through hole, thescanning head moves to a next through hole and continues to fill thethrough holes until completed.

In accordance with the present principles, the solder filling system andmethod permit through holes with a high aspect ratio to be completelyfilled. The high aspect ratio may include aspect ratios between 10 and25, although greater or lesser aspect ratios may be employed and benefitin accordance with the present principles.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, may be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the FIGs. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGs. For example, if the device in theFIGs. is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein may be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers may also be present.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Having described preferred embodiments for solder fill into high aspectthrough holes (which are intended to be illustrative and not limiting),it is noted that modifications and variations can be made by personsskilled in the art in light of the above teachings. It is therefore tobe understood that changes may be made in the particular embodimentsdisclosed which are within the scope of the invention as outlined by theappended claims. Having thus described aspects of the invention, withthe details and particularity required by the patent laws, what isclaimed and desired protected by Letters Patent is set forth in theappended claims.

What is claimed is:
 1. A method for filling a through hole withconductive solder, comprising: mounting a substrate having a throughhole formed therein on a permeable barrier layer having pores that havea pore size that enables gas to flow through the permeable barrier andprovides surface tension that prevents solder from flowing through thepermeable barrier; positioning a solder source over the through hole;and delivering molten solder in the through hole with a positivepressure from the solder source such that gas in the through holespasses the permeable barrier while the molten solder remains in thethrough hole, the solder being conductive.
 2. The method as recited inclaim 1, wherein the positive pressure is greater than 1 atmosphere. 3.The method as recited in claim 1, wherein the solder source includes arepositionable scanner head.
 4. The method as recited in claim 1,further comprising heating the permeable barrier and the substrate toensure the molten solder fills the through hole.
 5. The method asrecited in claim 4, wherein heating includes heating with a heatedplate.
 6. The method as recited in claim 5, further comprising apermeable plate disposed between the heated plate and the permeablebarrier that further permits gas flow through the permeable plate. 7.The method as recited in claim 1, wherein the through holes include anaspect ratio between 10 and
 25. 8. A method for filling one or morethrough holes with conductive solder, comprising: mounting a substratehaving a through hole formed therein on a permeable barrier layer havingpores that have a pore size that enables gas to flow through thepermeable barrier and provides surface tension that prevents solder fromflowing through the permeable barrier; pressurizing a solder reservoir;positioning a scanner head over the through hole configured to delivermolten solder from the solder reservoir; delivering molten solder in thethrough hole with a positive pressure such that gas in the through holespasses the permeable barrier while the molten solder remains in thethrough hole, the solder being conductive; and repositioning the scannerhead over another through hole.
 9. The method as recited in claim 8,wherein pressurizing the solder reservoir includes generating thepositive pressure of greater than 1 atmosphere.
 10. The method asrecited in claim 8, further comprising heating the permeable barrier andthe substrate to ensure the molten solder fills the through hole. 11.The method as recited in claim 10, wherein heating includes heating witha heated plate.
 12. The method as recited in claim 11, furthercomprising a permeable plate disposed between the heated plate and thepermeable barrier that further permits gas flow through the permeableplate.
 13. The method as recited in claim 8, wherein the through holesinclude an aspect ratio between 10 and 25.